The present invention relates to a developing device for applying a developer to the latent image formed on an image carrier. More particularly, the invention relates to a developing roller for applying a developer, while held in contact with a surface of an image carrier that is shaped like a belt.
Image forming apparatuses such as printers and copiers are known, each having a pair of rollers and a photosensitive belt wrapped around the rollers. In an image forming apparatus of this type, an electrostatic latent image is formed on a surface of the photosensitive belt. A developer is applied onto the surface of the belt, thereby developing the electrostatic latent image into a developer image. The developer image is transferred onto a recording paper sheet and then fixed. An image is thereby formed on the recording paper sheet.
The image forming apparatus has a developing device designed to develop the electrostatic latent image on the recording paper sheet. The developing device has a developing roller for applying the developer onto the surface of the photosensitive belt. The developing roller is positioned, with its circumferential surface contacting the surface of the photosensitive belt. In operation, the developing roller is rotated in the same direction as the belt is driven and at a speed a little higher than the belt is driven. Therefore, the developing roller slides on the photosensitive belt.
The developing roller is mirror-surfaced at both end portions. The middle portion of the developing roller has a circumferential surface that is roughened to hold the developer. Thus, the developing roller can hold the developer on its middle portion, with its end portions pressed onto the surface of the photosensitive belt and thus kept clear of the developer. The developer on the middle portion of the developing roller never flows to the end portions or fall from therefrom in drops.
As the developing roller slides on the photosensitive belt, however, those parts of the belt which contact the mirror-surfaced end portions may be worn. This is because nothing acts as lubricant between the belt and the end portions of the developing roller and a prominent friction occurs between the belt and the end portions. Note that the developer works as lubricant between the belt and the middle portion of the developing roller.
The photosensitive belt comprises an endless, thin metal band and a photosensitive coating provided on one surface of the metal band. The developing roller applies the developer to the photosensitive belt, while a developing bias voltage is being applied between the belt and the developing roller. If the photosensitive coating is scraped, the metal band will be exposed and a leakage current will flow. The leakage current may damage the photosensitive belt.
The present invention has been made in view of the foregoing. The object of the invention is to provide a developing roller and a developing device, which can minimize wear of an image carrier that remains in frictional contact with the developing roller, thereby increasing the lifetime of the image carrier.
To achieve the object, a developing roller according to the invention has an outer circumferential surface for holding a developer and designed to rotate apply the developer to a belt-shaped image carrier while rotating with the circumferential surface set in frictional contact with the image carrier. The developing roller comprises: two developer-non-retaining parts provided near the ends of the roller, for retaining no developer; a developer-retaining part extending between the two developer-non-retaining parts, for retaining the developer; and spacer means for preventing the image carrier from coming to frictional contact with the two developer-non-retaining parts.
A developing device according to the invention comprises: a reservoir containing a developer; a developing roller having an outer circumferential surface for holding a developer and designed to rotate apply the developer to a belt-shaped image carrier while rotating with the outer circumferential surface set in frictional contact with the image carrier; a developer-applying section for applying the developer from the reservoir to the outer circumferential surface of the developing roller. The developing roller comprises: two developer-non-retaining parts provided near the ends of the roller, for retaining no developer; a developer-retaining part extending between the two developer-non-retaining parts, for retaining the developer; and spacer means for preventing the image carrier from coming into frictional contact with the two developer-non-retaining parts.